1. Field of the Invention (Technical Field)
The present invention relates to apparatus for measuring, detecting, and imaging surface properties, for example, surface resistance and corrosion in various materials, including superconductors and normal conductors; and a method of using the apparatus.
2. Background Art
As disclosed in U.S. Pat. No. 5,239,269, and the publication entitled "Confocal Resonators for Measuring the Surface Resistance of High Temperature Superconducting Films," by J. S. Martens, V. M. Hietala, D. S. Ginley, T. E. Zipperian, and G. K. G. Hohenwarter, Appl. Phys. Lett., Vol. 58, p. 2543 (Jun. 3, 1991), the advent of numerous new, relatively high critical temperature, superconducting alloys and materials has brought with it a commensurate need to measure various properties of such materials, such as surface resistance and consistency. For example, U.S. Pat. No. 4,873,444, entitled Detection of Surface Impurity Phases in High-Tc Superconductors Using Thermally Stimulated Luminescence, to Cooke, et al., discloses a process of detecting surface impurities by irradiating a sample with ionizing radiation, heating the irradiated sample to luminescence, and comparing the integrated luminescence with a calibration curve to determine surface resistance. This process, however, necessarily suffers from the disadvantage of testing the samples under normal conducting states, rather than under operational superconducting states.
"Morphology Control and High Critical Currents in Superconducting Thin Films in the Tl--Ca--Ba--Cu--O System," by D. S. Ginley, et al., Physica C, Vol. 160, pp. 42-48 (1989), discusses superconducting polycrystalline thin films in the Tl--Ca--Ba--Cu--O system with extremely high transition temperatures.
R. W. Keyes, et al., discuss the suggested advantages and possible disadvantages of operative logic circuitry at low temperatures in "The Role of Low Temperatures in the Operation of Logic Circuitry," Proceedings of the IEEE, Vol. 58, No. 12, pp. 1914-1932 (December 1970).
A. L. Schawlow, et al., in "Infrared and Optical Masers," Physical Review, Vol. 112, Number 6, pp. 1940-1949 (December 1958), discuss maser oscillation using resonant cavities of microwave dimensions.
K. E. Lonngren, et al., in an article entitled "On the Focused Fabry-Perot Resonator in Plasma Diagnostics," appearing in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-12, pp. 548-549 (1964), discuss plasma diagnostics using a Fabry-Perot resonator at frequencies lower than microwave frequencies.
Wavemeter cavities for measuring surface resistance of superconductors are also known in the prior art, as disclosed in "Microwave Surface Resistance of YBa.sub.2 Cu.sub.3 O.sub.6.9 Superconducting Films," Appl. Phys. Lett., Vol. 52, No. 21, 23 pp. 1822-1824 (May 1988). While this process permits surface resistance measurement under superconducting conditions, the sample must be in contact with the cavity itself, complicating the measurement process and requiring additional energy to maintain cryogenic conditions.
Confocal resonators, that is, resonators comprising two spherical mirrors with the center of curvature of each mirror at the other spherical surface, are also known in the prior art. Such resonators are known in the prior art, for example, for measuring dielectric constants of various insulating materials. "The Effects of Processing Sequences on the Microwave Surface Resistance of TlCaBaCuO", by J. S. Martens, T. E. Zipperian, D. S. Ginley, V. M. Hietala, C. P. Tigges, and T. A. Plut, J. Appl. Phys., Vol. 69, p. 8268 (Jun. 15, 1991), discloses the effects of various typical processing techniques on the surface resistance of TlCaBaCuO supercooling thin films.
It is also desirable to detect corrosion and/or thin layer growth or removal on conductors at an early state (before such are visible) for an understanding of these processes and for optimizing corrosion resistance and material modification. The development of such a technique allows repairs or structural analysis to take place before the damage is catastrophic. Further, such a technique will provide a powerful probe of the kinetics and thermodynamics of the growth or etching process. Structures on which this technique could be used include any conductor where the corrosion or thin film growth has some real component of conductivity. Materials of interest include, for example, circuit board metallizations, structural members in aircraft and other assemblies, and microelectronic pad structures. Of particular interest are the degradation mechanisms in the high temperature superconducting (HTS) ceramic oxides in which the surface chemistry is very complicated. A simple corrosion detection technique would also be useful in evaluating the effects of various chemicals on material surfaces.
Currently, the analysis of corrosion and many thin film growth processes relies on electrochemical or optical probing of the interface. Electrochemical techniques, such as disclosed in U.S. Pat. No. 4,800,165, to Tomoki Oka, et al., entitled Method of Detecting Corrosion Rate of Member of Steel Material, are invasive, requiring contact to the sample and the use of an electrolyte. They are predominantly measurements of the cell potential, capacitance, or photoelectrochemical behavior. In situ diagnostics are not possible in the majority of the electrochemical methods, and the analysis of the surfaces of large objects, such as aircraft or boats is not possible. Optical techniques can be employed in situ, but with the exception of a few approaches, such as electroreflectance and ellipsometry, such as disclosed in "Hydrogen Plasma Treatment of Silicon Surfaces Studied by In-Situ Spectroscopic Ellipsometry," by P. Raynaud, J. P. Booth, and C. Pomot, Applied Surface Science, Vol. 46, pp. 435-440, (1990), they cannot be used on the thin initial layer due to insufficient absorption. Also, in many cases, the active chemical environment during corrosion or thin film growth processes precludes the use of optical probes. Other techniques such as surface analytical methods typically require high vacuum and cannot be used in situ or as a probe of process thermodynamics or kinetics. None of these approaches provide a rapid, non-invasive technique for sample characterization.
The confocal resonator technique can be used ex situ or in situ to probe any layer which changes the surface resistance of the sample. Since it can look through a window or be used in air, a wide range of applications is possible. Its portability means it can be used to scan large, irregular surfaces. The confocal technique is non-destructive and requires no special samples.